1. Field of the Invention
This invention relates to solvent-based separation of minerals from mineral feedstock, and more particularly relates to extracting bitumen from tar sands.
2. Description of the Related Art
Separation of bitumen from tar sands is known in the current art. The currently available technologies suffer from a number of drawbacks—low yield of bitumen recovery, environmental issues with waste water disposal, environmental issues with sand disposal, release of solvent vapors to the atmosphere, release of hydrocarbons to the atmosphere, sensitivity to clays, sensitivity to oil-wet tar sands, generation of emulsions during separation, high energy input, clogging of sand draining screens, and clogging of the valves that manage counter-current flow.
Most of the processes used in the current art are some variation of the Clark hot water process. One common variation of this process is to run mineral feedstock up a a partially vertical screw feeder. The mineral feedstock is run through a solvent layer, then a water layer.
The solvent-hydrocarbon miscella formed is denser than water and must be extracted below the water layer. The fluid levels and extraction rates must be carefully controlled, or water will be drawn into the miscella extraction apparatus. The fluid layers are not stable in such systems. Any hydrocarbons that are in a miscella without enough solvent portion will float to the top of the contact chamber. This means that some hydrocarbon will be floating to the top of the system regardless of the design, and that the extracted miscella must be solvent-rich rather than hydrocarbon-rich so that the miscella doesn't float. The separation of solvent-rich miscella is more energy intensive than the separation of hydrocarbon-rich miscella.
An additional water layer serves as a cap to contain the organic solvent in the solvent-sand mixing chamber of such systems. That exposes the sand-solvent mixture to water. Water exposure of the sand-solvent mixture can swell clays, flocculate the mineral feedstock, and create emulsions within the sand-solvent mixture. All of these effects complicate the separation process.
The process allows only a single solvent-feedstock contact, the solvent-hydrocarbon miscella composition must be kept within a narrow range of compositions, and the waste water from these systems cause environmental complications. Overall, this process provides an inflexible solvent contact method and produces low bitumen recovery from the mineral feedstock—typically on the order of 50%.
Another process in the current art is to run mineral feedstock up a partially vertical screw feeder and run solvent without water in counter flow with the sand. Solvent flow is usually controlled in these systems with reed valves that get plugged, and stuck partially open with sand and are therefore high maintenance. Another solvent flow control employs tortuous slots in the flights of the screw feeder which allow liquid but not solids to pass. This mechanism complicates control of the contact time of the solvent with the mineral feedstock, and the contact times between the solvent and the mineral feedstock tend to be short as the solvent gravity feeds through the system. In addition, the slots become clogged with fines from the mineral feedstock. The clogging causes poor solvent-feedstock contact, and is a complicated maintenance problem to both diagnose the occurrence of the clogging, and to shut the system down to fix the clogging.
Overall, this process is a high maintenance process which produces low bitumen yields because the solvent-feedstock contact times are difficult to control. The counter flow nature of these processes is better than the single pass contact of the typical Clark hot water implementation, but is still not as controllable. Much of the solvent-feedstock contact occurs at the end of the system where the miscella is hydrocarbon-rich. Consequently, this solvent-feedstock contact is low quality, and these systems must be large or they must be designed for a low hydrocarbon yield.
Another process in the current art is to run mineral feedstock along a continuous belt, while spraying solvent onto the sand at various points along the belt. The solvent picks up some fraction of the hydrocarbon material and drains through perforations in the belt. This process allows multiple contacts between fresh solvent and feedstock, but the contact occurs in a static feedstock environment, the contact time is minimal, and the contact time cannot be controlled because it relies on gravity. Because only limited amounts of hydrocarbon are stripped by the solvent, the process requires some combination of: significant amounts of fresh solvent, pumping significant amounts of recycled solvent, a large conveyor system, or a design for a low hydrocarbon yield. Further, the perforations in the belt tend to plug with fines from the mineral feedstock. The plugging of the perforations is a complicated maintenance problem to both diagnose the occurrence of the plugging, and to shut the system down to fix the plugging.
Finally, the current art depends upon passive containment to prevent escape of solvent vapors to the atmosphere. Typically, a water layer is kept on top of all otherwise exposed solvent layers. Where water is not used, solvent is exposed to the atmosphere through the sand feeder.
The state of the current art is perhaps best highlighted by the fiscal year 2005 United States Department of Energy solicitations for new technologies. Technical topic 12(d) is a request for Tar Sands and Oil Shale Development, wherein the Department requests a technology that leaves clean sands, leaves low organic content in the waste water, does not release excessive volatiles to the atmosphere, leaves minimal fines in the bitumen product, and that will not flocculate clays.
From the foregoing discussion, it should be apparent that a need exists for an apparatus, system, and method that separates minerals from mineral feedstock. Beneficially, such an apparatus, system, and method would produce clean sand, generate no waste water, have low atmospheric emissions, be adaptable to the clay content and wetting of the mineral feedstock, minimize mechanical complications, and have low energy input requirements.